“The smaller the planets are, they are, other things being equal, of so much the greater density; for so the powers of gravity on their several surfaces come nearer to equality. They are likewise, other things being equal, of the greater density, as they are nearer to the sun.”
- January 4, 1643 – March 31, 1727
- Born in England (UK)
- Natural philosopher, mathematician, physicist, astronomer, theologian
- Proposed the laws of universal gravitation and motion, built the foundations of modern science, and systematized theories of natural philosophy and mathematics.
Quote
“The smaller the planets are, they are, other things being equal, of so much the greater density; for so the powers of gravity on their several surfaces come nearer to equality. They are likewise, other things being equal, of the greater density, as they are nearer to the sun.”
Explanation
In this quote, Isaac Newton is discussing the relationship between a planet’s size, density, and its proximity to the Sun in the context of gravity. Newton points out that smaller planets tend to have a greater density (i.e., a higher mass per unit volume) compared to larger planets, assuming other factors are equal. The reason for this, he explains, is that as the size of a planet decreases, its gravitational force at the surface becomes stronger, which requires the planet to be denser to balance this force. This reflects the idea that the force of gravity on a planet’s surface is related to its mass and size; smaller planets must compensate for their size by having a denser composition, allowing the gravitational forces on their surfaces to remain relatively strong.
Newton also suggests that planets that are closer to the Sun are likely to have a greater density, again assuming other factors are equal. This idea may relate to the observation that terrestrial planets (like Mercury, Venus, and Earth), which are closer to the Sun, tend to be denser than the more gaseous giants (like Jupiter and Saturn) farther from the Sun. The greater density of planets near the Sun might be attributed to the higher temperatures and pressures in the inner solar system, which may result in a greater proportion of rocky or metallic materials compared to the outer, cooler regions of the solar system, where gas and ice are more prevalent.
In modern terms, Newton’s observations are consistent with our understanding of planetary formation and density variations in the solar system. The relationship between distance from the Sun, planet size, and density can be explained by differential condensation in the early solar system, where heavier elements condensed closer to the Sun, forming rocky planets, and lighter elements condensed further out, forming gas giants. Newton’s insights into gravity and density laid the groundwork for later developments in planetary science and astronomy, particularly in the study of how planets form and evolve based on their distance from the Sun and the materials available in different regions of the solar system.
